The present invention relates to a process for recovering gallium from smelting dust.
Gallium is used as GaAs for high-speed semiconductors for satellite correspondence, IC for high-speed computers, GaAs or GaP for semiconductor lasers for optical communication and mass magnetic bubble domain memories using gallium gadolinium garnet.
Gallium is widely distributed all over the earth, but does not exist as highly concentrated ore.
Therefore, gallium is presently obtained by the following processes
(i) Gallium is recovered from Bayer liquor, which is a highly concentrated alkaline solution dissolving Al(OH).sub.3. An electrochemical reduction process with a mercury cathode or a solvent extraction process is used for recovering gallium from the Bayer liquor. PA1 (ii) Gallium is recovered from aluminum smelting dust by flotation or alkaline roasting (See British Pat. No. 1527981). PA1 (iii) Gallium is recovered from residue of the zinc leach solution in a zinc smelting process, in which the residue is leached with sulfuric acid. Gallium is recovered by an extracting agent such as versatic acid or isopropyl ether (see pages 65-76, Proceeding of Fourth Joint Meeting MMIJ-AIME 1980, Tokyo). PA1 (a) leach liquor is obtained by leaching aluminum smelting dust with mineral acid; PA1 (b) gallium is extracted by extracting the leach liquor obtained in step (a) with an organic solution containing a liquid cation exchange agent; PA1 (c) impurities such as iron, vanadium and aluminum extracted together with gallium in the loaded organic solution are scrubbed off by scrubbing the loaded organic solution obtained in step (b) with mineral acid in aqueous solution having a higher concentration; PA1 (d) gallium is stripped from the loaded organic solution by stripping the loaded organic solution scrubbed in step (c) with mineral acid in aqueous solution having a lower concentration; PA1 (e) the acid concentration of the stripped liquor obtained after step (d) is adjusted to a higher concentration; PA1 (f) all gallium and a portion of the iron are extracted by extracting the stripped liquor obtained after adjusting the acid concentration in step (e) with another organic solution containing a neutral extracting agent or a basic extracting agent of the ion-pair extracting type; and. PA1 (g) gallium is stripped by stripping the loaded organic solution obtained in step (f) with water or dilute mineral acid solution.
The solvent extraction process developed by Rhone-Poulenc Co. using KELEX 100 (trade name) is wellknown (See pages 439-442, Vol. 2, 1979, Proceeding of the International Solvent Extraction Conference, . French Pat. No. 7629009), and KELEX 100 is an extracting agent such as alkylated-8-hydroxyquinoline.
Though gallium is recovered from the residue of a zinc leach solution (iii) in Japan, the major source of gallium is bauxite, which is the raw material for making aluminum. However, the efficiency of recovering gallium directly from bauxite is low because of the low gallium content in bauxite, e.g., 50-100 p.p.m.
The process for recovering gallium from Bayer liquor is mentioned in process (i) above. Electrochemical reduction with a mercury cathode creates an environmental pollution problem because of the mercury. The solvent extraction process also has problems, e.g., deterioration of the expensive extracting agent, loss by decomposition and the suspension and contamination of the Bayer liquor itself, with organics because the Bayer liquor is highly caustic.
The complicated processes and expensive treating costs are required in process (i) because of the very low gallium content. Accordingly, aluminum smelting dust having a comparatively high gallium content is preferable as the raw material. Gallium contained in Bayer liquor exhibits the same behavior as aluminum, . Almost all the gallium goes into alumina. The gallium concentration in alumina depends on the quality of the bauxite, and 10-100 p.p.m. is usual. Therefore, gallium in the dust created during the electrolysis by using alumina is highly concentrated to 800 p.p.m.-3000 p.p.m. The general composition of dust evolved from an aluminum electrolysis process is 0.08-0.30 wt% of gallium, 0.5-2.0 wt% of iron, 10-20 wt% of aluminum, 10-20 wt% of sodium, 10-30 wt% of flourine, 10-30 wt% of carbon and less than 1 wt% of silicon, nickel, titanium, calcium, copper, magnesium, cobalt and vanadium.
British Pat. No. 1527981 relates to a process for producing gallium from aluminum smelting dust containing, at the most, 0.2% gallium.
The process consists of roasting the dust at a temperature of 500.degree.-800.degree. C., after adding alkali flux in a large excess (5 times the dust weight, for example), followed by leaching the roasted mixture of dust and flux with water, and then precipitating the dissolved gallium by adding base metal powder (aluminum or magnesium) into the leach liquor.
However, the above-mentioned process has a great limitation, practically, because iron, or aluminum, etc., are also recovered togethe with the gallium at the same time, and more of the expensive alkaline flux is added than the amount of the dust for roasting. The cost of the alkaline flux may exceed the price of the gallium to be recovered.